This invention relates to a projection exposure apparatus and, more particularly, to a projection exposure apparatus suitably usable in a lithographic process, for the production of large-scale integration devices, wherein an electronic circuit pattern formed on the surface of a mask or reticle (hereinafter, xe2x80x9creticlexe2x80x9d) is printed on a wafer by projection through a projection optical system or by scanning exposure, for the manufacture of various devices such as semiconductor devices (e.g., ICs or LSIs), image pickup devices (e.g., CCDs), display devices (e.g., liquid crystal panels), or magnetic heads, for example.
In a photolithographic process for the production of semiconductor devices or liquid crystal panels, for example, a pattern formed on the surface of a reticle is transferred, by exposure and by use of a projection optical system, onto a photosensitive substrate such as a wafer or a glass plate on which a photoresist is applied.
The density of integrated devices such as ICs or LSIs is increasing more and more, and semiconductor wafer microprocessing techniques have been advanced to meet the same. Projection exposure apparatuses play a major role in such microprocessing techniques, and there are a unit-magnification projection exposure apparatus (mirror projection aligner) wherein a mask and a photosensitive substrate are exposed while being scanned with respect to a unit-magnification mirror optical system having an exposure region of an arcuate shape, and a reduction projection exposure apparatus (stepper) wherein an image of a mask pattern is formed on a photosensitive substrate through a dioptric system and the photosensitive substrate is exposed in accordance with a step-and-repeat method.
Recently, the resolving power of a projection optical system to be incorporated into a projection exposure apparatus has been increased more and more, and thus, very strict requirements have been applied to aberration correction for the projection optical system. For this reason, after a projection optical system is mounted on a major assembly of an exposure apparatus, in many cases, the optical performance of the projection optical system is measured and inspected.
For inspection of the performance of a projection optical system incorporated into a main assembly of an exposure apparatus, particularly, for the inspection of aberrations, a plurality of light blocking patterns (such as line-and-space patterns) are formed on a light transmitting portion of a reticle, and the reticle pattern is then actually printed on a wafer. A resist image thus formed is then observed by use of an electron microscope, for example, to perform the inspection.
The method of inspecting the optical performance of a projection optical system, by observing a resist image printed on a wafer with the use of an electron microscope, involves a complicated procedure such as an exposure process and a development process, to obtain a resist image. As a result, it needs a very long time for the whole inspection.
Further, it requires the use of a high precision measuring system for the inspection of a resist image. Currently, a scanning electron microscope (SEM) is the only inspection apparatus therefor. However, the measurement precision of the SEM is variable with the optical axis alignment precision of electron optical systems or the inside vacuum level thereof, for example. Thus, there is a possibility that a difference is produced in measured values, depending on individual skills of operators or the state of the apparatus, for example.
Additionally, because of inspection of a resist image formed on a wafer, the inspection precision will degrade largely if there is an error in a resist process (e.g., resist coating or development) and, particularly, the inspection reproducibility is lowered significantly in that case. Further, since the inspection printing has to be done with respect to each of the illumination conditions which might be used practically in device printing operations, heavy work is necessary for the inspection.
On the other hand, in accordance with further miniaturization of devices, very strict requirements are applied with respect to maintaining the optical performance of a projection optical system. For example, in some cases, a small change in optical performance of a projection optical system caused during transportation of the same has to be measured at the time of set-up, and re-adjustment has to be done after the set-up to assure the best optical performance.
Also, it is desired to minimize a change in the optical performance of a projection optical system due to a change in illumination condition. To this end, it is desired that the performance of a projection optical system in various conditions, as the same is mounted on a main assembly of an exposure apparatus, can be measured conveniently.
Practically, during an exposure process, a projection optical system is warmed by an illumination system and, as a result, the image performance may be changed thereby. Conventional exposure apparatuses are not equipped with any means for performing image evaluation or measurement of wavefront aberration of a projection optical system, and they have no adjusting means therefor. The only way to meet the same is to interrupt the operation of the apparatus.
It is accordingly an object of the present invention to provide a projection exposure apparatus having an interferometer, by which an optical performance of a projection optical system can be measured and inspected conveniently and precisely in a short time, upon a main assembly of a projection exposure apparatus and under various illumination conditions.